DESCRIPTION: [From Applicant's Abstract] Neurotransmitter receptors mediate signal transduction at the postsynaptic membrane of synaptic connections in the central and peripheral nervous systems. A variety of data now suggest that protein phosphorylation is a primary mechanism for the regulation of neurotransmitter receptor function and may play a major role in the modulation of synaptic transmission. Today's understanding of the molecular mechanisms underlying synapse formation and function derives primarily from studies of the neuromuscular junction (NMJ). At the NMJ, the acetylcholine receptor (AChR) is the ligand gated ion channel that mediates the rapid postsynaptic response of the muscle. The AChR is phosphorylated on tyrosine residues. Tyrosine phosphorylation of the AChR is believed to regulate channel desensitization rate as well as AChR clustering during synaptogenesis. Two Src like kinases, Fyn and Fyk, that account for the predominate protein tyrosine kinase activity in the nicotinic postsynaptic membrane have been identified. Fyn and Fyk phosphorylate the AChR in vitro and form a high affinity association with the receptor via a binding of their SH2 domains to the tyrosine phosphorylated subunit of the AChR. These data support the relevance of these kinases in synaptic function at the NMJ. The goal of this proposal is to continue to test the hypothesis that Fyn and Fyk are involved in synapse formation and function at the NMJ by regulating the AChR. In addition, the molecular mechanisms by which these two kinases regulate synaptic transmission will be investigated. The action of extracellular factors including agrin, ARIA, and bFGF to activate Fyn and Fyk will be tested. In addition, investigation of the signal transduction pathways by which Fyn and Fyk act to regulate the NMJ will be initiated by identifying molecular components that directly interact with the two kinases. Furthermore, the effect of innervation on the expression and synaptic localization of the kinases will be determined. Finally, the applicants will investigate the physiological roles of Fyn and Fyk in the control of AChR clustering and desensitization. Clarification of these basic molecular mechanisms will provide an understanding of synaptic transmission in healthy people as well as those afflicted with neurological and neuromuscular diseases.